1. Field of the Invention
The present invention relates to a communications system, a network device, and a method of reserving bandwidth resources. More particularly, the present invention relates to a Resilient Packet Ring communication system that transports packets over a redundant ring network. The present invention also relates to a network device, as well as to a method of reserving bandwidth resources, for use in an RPR system.
2. Description of the Related Art
Ring network systems based on the Synchronous Optical Network (SONET) or Synchronous Digital Hierarchy (SDH) standards have been the mainstream architecture of long-haul backbones for wide-area network service. Another technology called “Resilient Packet Ring” (RPR) is gaining interest in these years as an alternative to SONET/SDH systems. RPR is a new data transmission technique currently in the process of standardization by an IEEE committee. The IEEE 802.17 RPR standard offers protocols of Media Access Control (MAC) sub-layer, part of layer 2, like the Ethernet (registered trademark of Xerox Corporation) in LAN environments. RPR technology takes advantage of a ring topology, combined with an existing technique for layer 1.
RPR assumes the use in a metropolitan area network (MAN). It is possible to construct an RPR network on an existing backbone with a hierarchy of transmission rates, such as Optical Carrier (OC-n) of SONET networks or Synchronous Transport Module (STM-n) of SDH networks. 10-Gigabit Ethernet (10 GbE) may also be used as another option for layer 1 (physical layer) architecture. Such existing ring networks carry IEEE802.17 MAC frames (or RPR frames), thereby realizing “RPR over SONET/SDH,” “RPR over GbE,” or the like.
FIG. 10 provides an overview of an RPR network. This RPR network 100 includes four stations S1 to S4 and fiber optic links interconnecting those stations in a dual ring topology. Part of data traffic traveling over the ring network is dropped (i.e., split off) to tributaries at those stations S1 to S4. The stations S1 to S4 also allow incoming data traffic from tributaries to be added to the main data traffic on the ring.
The dual RPR ring consists of two unidirectional ringlets, Ringlet0 and Ringlet1, to transport packets in opposite directions. In the example of FIG. 10, Ringlet0 runs counterclockwise while Ringlet1 runs clockwise. Ringlet0 and Ringlet1 serve as a working system and a protection system, respectively. RPR networks transport and deliver data in “packets,” whereas SONET/SDH networks do the same in “streams” each accommodating a plurality of OC or STM channels.
The RPR architecture permits packets to have different classes for bandwidth control purposes. FIG. 11 enumerates RPR classes of service. Specifically, there are three class definitions: Class A, Class B, and Class C. Class A offers bandwidth-guaranteed service using previously reserved bandwidth resources for packet transport. This class of service minimizes end-to-end delays and jitters.
More specifically, Class A is divided into two subclasses A0 and A1. Of all classes, Class A0 packets enjoy the highest priority. Bandwidth reserved for Class A0 is for exclusive use by Class A0 services; that is, two or more Class A0 paths can use the same reserved bandwidth, but other classes of service cannot.
Class B is also divided into two subclasses: Class B-CIR (Committed Information Rate) and Class B-EIR (Excess Information Rate). Both Class A1 and Class B-CIR provide bandwidth-guaranteed services, but their reserved bandwidth resources may be used by other classes (i.e., they are for non-exclusive use). Class B-EIR and Class C (Class C-EIR), on the other hand, do not guarantee the bandwidth that they claim to offer. Instead, those classes offer best-effort transport service using remaining bandwidth.
The system operator provisions, if necessary, a new Class A0 bandwidth for a station, taking into consideration every existing Class A0 path on each ring. The operator performs this task by using his/her terminal console to send bandwidth configuration commands to that station. The receiving station then notifies every peer station of the provisioned bandwidth, so that the Class A0 bandwidth for that station will be reserved throughout the ring network.
FIG. 12 shows a conventional way of reserving bandwidth. The illustrated RPR network 110 is formed from six stations S1 to S6 interconnected by fiber optic links in a dual ring topology. The two ringlets, named Ringlet0 and Ringlet1, transport packets in the counterclockwise and clockwise directions, respectively.
Suppose now that a new path P1 has to be added to transport Class A0 packets from station S3 to station S4 at 100 Mbps. The system operator enters commands to his/her terminal console 103 to configure the ingress station S3, so as to provision a 100-Mbps bandwidth resource for the new path P1. The provisioning of this 100-Mbps bandwidth is reported to all other stations, thus reserving 100 Mbps for Class A0 service on both Ringlet0 and Ringlet1.
For an example of such an existing RPR-based technique, see Japanese Patent Application Publication No. 2003-249940, paragraph numbers 0008 to 0010, FIG. 1. This publication discloses a technique for realizing dynamic multicast routing control with a reduced signal processing workload.
One problem of the above-described conventional techniques for RPR bandwidth reservation is their inefficient use of bandwidth resources. Specifically, the existing techniques reserve extra bandwidth on both Ringlet0 and Ringlet1 when adding a new path P for Class A0 traffic. FIG. 13 shows this problem with conventional bandwidth reservation. In the illustrated RPR network 110, two hosts H3 and H4 are connected to stations S3 and S4, respectively. These hosts H3 and H4 belong to a first virtual LAN (VLAN) domain, VLAN1. VLAN is a logical network segment in which stations can communicate with each other as if they were in a physically closed network. There is another host H2 connected to station S2. The hosts H2 and H3 belong to a second virtual LAN domain, VLAN2, meaning that they can communicate with each other as if they were in another closed network.
VLAN1 uses a path P1 from station S3 to station S4, while VLAN2 uses another path P2 from station S2 to station S3. Suppose now that the first path P1 needs 100 Mbps for its Class A0 traffic, and that the second path P2 needs 50 Mbps for its Class A0 traffic. Suppose also that the ringlets have a capacity of 100 Mbps for each.
Both VLAN1 path P1 and VLAN2 path P2 are used as working paths. Since P1 does not overlap with P2, the above bandwidth requirements are supposed to be satisfied theoretically. That is, the network must be able to transport packets from S3 to S4 at 100 Mbps concurrently with another packet traffic from S2 to S3 at 50 Mbps.
The conventional bandwidth reservation techniques, however, allocates 100-Mbps bandwidth, not only to the first path P1 between S3 and S4, but also to the other links on Ringlet0 and Ringlet1, including the second path P2 between S2 and S3. The second path P2 with a capacity of 50 Mbps cannot be established because the bandwidth resource of the link between S2 and S3 has already been exhausted.
As can be seen from the above discussion, the conventional network system allocates its bandwidth on an entire ring basis, rather than reserving path bandwidth on an individual link basis. Such inefficient use of bandwidth spoils operability of the ring network.